This invention relates to a semiconductor laser, and in particular, to an AlGaInP based visible light semiconductor laser which is used as a light source for an optical disk, such as, a digital versatile disk (DVD) and a magneto-optical (MO) disk.
A ridge waveguide type laser illustrated in FIG. 1 is generally used in the conventional AlGaInP based visible semiconductor laser. The laser having such a structure is fabricated by performing the known MOVPE (Metalorganic Vapor Phase Epitaxy) method three times. For example, this method is disclosed in Japanese Unexamined Patent Publications No. Hei. 2-58883 and No. Sho. 62-200786.
A double hetero structure having an active layer is formed in a first crystal growth. Subsequently, the ridge structure of a mesa-stripe shape is formed by etching with a dielectric mask.
In a second crystal growth, a GaAs current block layer is formed on a clad layer outside the mesa-stripe by the use of the selective growth method using the same dielectric mask.
Finally, in a third crystal growth, a p-GaAs contact layer is formed on an entire surface, and the laser structure is formed after forming an electrode.
As illustrated in FIG. 1, the semiconductor laser has a n-GaAs buffer layer 170, a n-AlGaInP clad layer 130, a MQW active layer 110, a p-AlGaInP clad layer 120, a p-GaInP etching stopper layer 140, a n-GaAs block layer 180, a p-AlGaInP clad layer 150, a p-GaInP hetero buffer layer 160, a-pGaAs cap layer 190, a p-GaAs contact layer 195 on a n-GaAs substrate 200. Further, electrodes 210 and 220 are arranged at upper and lower sides of the semiconductor laser.
Referring to FIG. 2, description will be made about the conventional crystal growth apparatus for manufacturing such a semiconductor laser device by the use of the MOVPE method.
A susceptor 320, which mounts a GaAs substrate 330, is contained in a reactive tube 310. Further, a V-group raw material gas inlet 340, a hydrogen gas inlet 360 and a III-group three gas inlet 370 are arranged in the reactive tube 310.
In this event, organic metal (TMA1, TEGa, TMIn) is supplied as the III-group raw material of the semiconductor laser while hydrogenated V-group gas (PH.sub.3, AsH.sub.3) is supplied as the V-group raw material. Herein, hydrogen is supplied to the reactive tube 310 as a carrier gas.
In this case, attention is paid for atmosphere gas and growth temperature in a temperature dropping process after each crystal growth.
As illustrated in FIG. 3, mixed atmosphere between hydrogenated V-group gas, which is used for the growth of the final growth layer, and hydrogen gas as the carrier gas is generally used as the atmosphere gas. Herein, AsH.sub.3 gas is, for example, used as the hydrogenated V-group gas when the GaAs is the final layer.
However, the mixed atmosphere is decomposed during the temperature dropping process. Consequently, much active hydrogen exists in the atmosphere.
On the other hand, p-type impurities are inactivated by combining with hydrogen molecules in the crystal growth of the p-type semiconductor crystal of the AlGaInP based compound semiconductor. In consequence, p-carrier concentration is lowered.
The hydrogen is supplied from the atmosphere gas into the semiconductor crystal during the crystal growth and the temperature dropping process in the conventional method. Thereby, the crystal growth is completed on the condition that much hydrogen is left in the p-type semiconductor crystal (p-type impurity passivation region 260).
As a result, carrier-over increase, which causes reduction of the p-carrier concentration, occurs in the semiconductor laser manufactured by the conventional method. Consequently, deterioration of high temperature operation characteristic becomes remarkable.
Further, hydrogen, which is trapped into the crystal, is readily changed in the state with time. In consequence, it is difficult to realize stable laser characteristic.
Meanwhile, disclosure has been made about the atmosphere gas during the growth of the semiconductor in Japanese Unexamined Patent Publication No. Hei.8-32113 (hereinafter, referred to as a first reference). The first reference discloses a method of manufacturing a p-type GaN based semiconductor device.
More specifically, description has been made at page 1, ninth paragraph in the first reference about a technique featured by cooling at slower speed than natural cooling, preferably under an inactive gas atmosphere after a GaN based semiconductor, which is doped with a p-type impurity, is grown.
However, no description has been made about manufacture of the semiconductor laser in the first reference. Further, the first reference is limited to the GaN based semiconductor. Therefore, the first reference can not be applied for the semiconductor laser, such as, the AlGaInP based semiconductor laser.